It is a very hard, white to off-white, highly mineralised substance that acts as a barrier to protect the tooth but can become susceptible to degradation, especially by acids from food and drink.

[1] Enamel is the hardest substance in the human body and contains the highest percentage of minerals (at 96%),[2] with water and organic material composing the rest.

In humans, enamel varies in thickness over the surface of the tooth, often thickest at the cusp, up to 2.5 mm, and thinnest at its border with the cementum at the cementoenamel junction (CEJ).

At the edges of teeth where there is no dentin underlying the enamel, the color sometimes has a slightly blue or translucent off-white tone, easily observable on the upper incisors.

[6] Tooth enamel ranks 5 on Mohs hardness scale (between steel and titanium) and has a Young's modulus of 83 GPa.

[8] Enamel does not contain collagen, as found in other hard tissues such as dentin and bone, but it does contain two unique classes of proteins: amelogenins and enamelins.

While the role of these proteins is not fully understood, it is believed that they aid in the development of enamel by serving as a framework for minerals to form on, among other functions.

[10] In permanent teeth, the enamel rods near the cementoenamel junction (CEJ) tilt slightly toward the root of the tooth.

[11] Perikymata which are associated with the Striae are shallow grooves noted clinically on the nonmasticatory surfaces of some teeth in the oral cavity.

[6] Perikymata are usually lost through tooth wear, except on the protected cervical regions of some teeth, especially the permanent maxillary central incisors, canines, and first premolars, and may be confused as dental calculus.

Human enamel forms at a rate of around 4 μm per day, beginning at the future location of cusps, around the third or fourth month of pregnancy.

[14] When this first layer is formed, the ameloblasts move away from the dentin, allowing for the development of Tomes' processes at the apical pole of the cell.

[23] Sugars and acids from candies, soft drinks, and fruit juices play a significant role in tooth decay, and consequently in enamel destruction.

When acids are present and the critical pH is reached, the hydroxyapatite crystallites of enamel demineralize, allowing for greater bacterial invasion deeper into the tooth.

[25] Furthermore, tooth morphology dictates that the most common site for the initiation of dental caries is in the deep grooves, pits, and fissures of enamel.

When demineralization of enamel occurs, a dentist can use a sharp instrument, such as a dental explorer, and "feel a stick" at the location of the decay.

For example, in terms of oral health, it is better to eat a single dessert at dinner time than to snack on a bag of candy throughout the day.

[citation needed] Though enamel is described as tough, it has a similar brittleness to glass, making it, unlike other natural crack-resistant laminate structures such as shell and nacre, vulnerable to fracture.

In between two adjacent teeth, floss is used to wipe the enamel surfaces free of plaque and food particles to discourage bacterial growth.

Although neither floss nor toothbrushes can penetrate the deep grooves and pits of enamel, good general oral-health habits can usually prevent enough bacterial growth to keep tooth decay from starting.

[15] Fluoride catalyzes the diffusion of calcium and phosphate into the tooth surface, which in turn remineralizes the crystalline structures in a dental cavity.

The combination of fluoride ions and QAS (quaternary ammonium salts) was found to have a stronger antimicrobial effect on many oral bacteria associated with dental decay, including S. mutans.

Most dental professionals and organizations agree that the inclusion of fluoride in public water has been one of the most effective methods of decreasing the prevalence of tooth decay.

Fluorosis is a condition resulting from the overexposure to fluoride, especially between the ages of 6 months and 5 years, and appears as mottled enamel.

In the past, the process of placing dental sealants involved removing enamel in the deep fissures and grooves of a tooth, followed by replacing it with a restorative material.

In spite of this, there are still cases where deep fissures and grooves in enamel are removed in order to prevent decay, and a sealant may or may not be placed depending on the situation.

[45] However, a tooth whitening product with an overall low pH can put enamel at risk for decay or destruction by demineralization.

Celiac disease, a disorder characterized by an auto-immune response to gluten, also commonly results in demineralization of the enamel.

[59] Enamel or enameloid is found in the dermal denticles of sharks and many early vertebrates,[60] and it appeared there before gnathostome teeth evolved.

Single crystallite hydroxyapatite has a higher hardness and young's modulus, which may be due to the defects present in enamel, such as substitutional ions as well as the presence of organic materials.